1. Technical Field of the Invention
The present invention relates to the separation of at least one aliphatic diacid from immixtures thereof with adipic acid.
2. Description of the Prior Art
Adipic acid is one of the two basic starting materials for the production of nylon-66 (polyamide-66).
Various processes are known to this art for the preparation of adipic acid, certain of which are industrial and others are at the research or development stage.
One process described in the patent literature entails hydroxycarbonylating butadiene into pentenoic acids, and then hydroxycarbonylating said pentenoic acids into adipic acid. During the second hydroxycarbonylation reaction, methylglutaric acid, ethylsuccinic acid and dimethylsuccinic acid are also formed, in amounts which vary depending on the conditions under which the process is carried out.
These various branched saturated aliphatic diacids may be upgradable to a greater or lesser extent, but it is important in each instance to separate them as completely as possible from the adipic acid, which remains, both qualitatively and quantitatively, the most commercially important compound.
Processes for fractionating these mixtures can thus be employed, for example by crystallization, to separate the branched diacids from the adipic acid. However, in respect of the adipic acid isomers, this technique does not permit an efficient, easy separation, because of the very close physical properties of all of these diacids.